Implementations of `Reader` now must implement `Reader::seekable`, and `AsyncSeekForward` is deleted.

The Reader trait no longer requires implementing AsyncSeekForward and instead requires implementing Reader::seekable, which will cast the Reader to &mut dyn SeekableReader if it supports AsyncSeek (SeekableReader: Reader + AsyncSeek).

// If MyReader implements `AsyncSeek` 
impl Reader for MyReader {
    fn seekable(&mut self) -> Result<&mut dyn SeekableReader, ReaderNotSeekableError> {
        Ok(self)
    }
}

// If MyReader does not implement `AsyncSeek` 
impl Reader for MyReader {
    fn seekable(&mut self) -> Result<&mut dyn SeekableReader, ReaderNotSeekableError> {
        None
    }
}

Since we now just use the AsyncSeek trait, we've deleted the AsyncSeekForward trait. Users of this trait can migrate by calling the AsyncSeek::poll_seek method with SeekFrom::Current(offset), or the AsyncSeekExt::seek method.